This invention relates broadly to the preparation of a novel thickened liquid carbon dioxide (CO.sub.2) composition. More particularly, the invention is concerned with the use of a thickened liquid CO.sub.2 compostion in the treatment of subterranean fluid-bearing formations to stimulate natural gas production.
Numerous methods are known in the industry for increasing the productivity of oil and gas wells in reservoirs having low permeability. A method now frequently employed is to inject liquid CO.sub.2 into the subterranean formation adjacent the well bore. This liquefied gas, often containing a propping agent such as sand, enters the small fissures adjacent the well bore where it vaporizes and expands to fracture the formation thereby allowing trapped oil or gas to flow towards the well bore. U.S. Pat. No. 3,368,627, for example, describes the use of a mixture of liquefied CO.sub.2 and a liquefied hydrocarbon such as propane as a fracturing fluid. In U.S. Pat. No. 3,310,112, the use of a gelled hydrocarbon slurry or emulsion in admixture with liquid CO.sub.2 is proposed for the same purpose. U.S. Pat. No. 3,396,107 describes a fracturing composition consisting of a liquid CO.sub.2 and water mixture. U.S. Pat. No. 3,623,552 describes a well displacement process wherein liquid CO.sub.2 is injected into a formation in three phases or stages each phase having a different density. In U.S. Pat. No. 3,664,422, the use of liquid CO.sub.2 in combination with a gelled alcohol is proposed for use in a well treating system. U.S. Pat. No. 3,765,488 discloses the use of a fracturing fluid comprising liquid CO.sub.2 mixed with a gelled alcohol, using a specific hydroxyalkyl cellulose gelling agent. In U.S. Pat. No. 3,842,910, a particular well-treating method making use of liquid CO.sub.2 is described. U.S. Pat. No. 3,954,626 provides a well-treating composition comprising liquid CO.sub.2, alcohol and a hydroxy-propyl methylcellulose gelling agent.
While all of the above noted inventions are meritorious, none has completely alleviated the problems associated with the use of liquid CO.sub.2 as a well-fracturing fluid. Although liquid CO.sub.2 is a near ideal fracturing fluid, since it vaporizes in situ, it nevertheless has a very low viscosity and, hence, must be combined with, for example, a gelled alcohol or similar material in order to support a propping agent and to provide a suitable fracture width. The presence of, for example, hydroxyalkyl cellulose thickening agents, however, often result in the deposit of an undesirable residue in the rock fissures. Furthermore, hydroxyalkyl cellulose thickeners are crosslinked only with difficulty and many species are not compatible with liquid CO.sub.2 Foaming of the liquid mixtures during pumping may also present problems when such thickeners are present. The description "thickened liquid CO.sub.2 " found in the prior patent literature tends to be misleading since it is the alcohol which is gelled or thickened and which is then diluted by the presence of the liquid CO.sub.2. What is required in the industry is a well-fracturing fluid which is sufficiently viscous to be delivered under pressure into a subterranean formation at a high pumping rate, which will not boil or foam during pumping, which carries a suspended propping agent without difficulty and which will completely degrade in the underground location without leaving any interfering solid or liquid residues in the formation.